1. Field of the Invention
This invention is directed to wear indicators for brakes, particularly air pressure actuated brakes or brakes actuated by a spring.
2. Description of Related Art
Common air-pressure-activated brakes have an actuator which includes an air pressure chamber and a flexible diaphragm or pistons linked mechanically to the brake by an actuator rod (called a "push rod"). The push rod is connected to a wedge, disc, cam or similar means which in turn is connected to the brake shoes (pads) The brake is actuated by the admission of air under pressure into the air pressure chamber or by releasing air off of a spring, forcing the diaphragm or piston to move and thereby moving the push rod. Movement of the push rod results in movement of the mechanism which in turn produces friction between the lining, disc, pad or drum to effect braking of the vehicle. A spring bearing against the push rod returns it to its normal non-braking position when the air pressure on the diaphragm or piston is relieved or air pressure is applied to compress or release a parking spring.
The distance a push rod travels is called its stroke. Push rod stroke distance can be altered in a variety of ways; e.g. brake shoes out of adjustment, worn brake shoes, or worn brake linings. When a push rod moves a relatively excessive amount, brakes should be adjusted or the shoes or linings be replaced. Brake shoes can become so worn that even when a push rod moves a maximum allowable distance, the shoes do not adequately contact the drums or discs to stop the vehicle. Such brake failure results in costly accidents, physical injury, and lost lives.
Common practices for checking brake wear include one person actually observing the travel of the push rod while someone else applies the brakes or forcibly extending the push rod against its return spring. A variety of solutions to the problem of indicating and inspecting push rod stroke have been attempted Nevertheless, brake failures still occur due to failure to note brake wear based on push rod stroke. The proposed devices are either difficult to inspect or too complex to be practical. Often indicators have been proposed which are exposed beneath a vehicle; hence they become covered with dirt and grime and, rather than having them cleaned off to obtain a good indication, they are ignored. Other indicators give false indications when brake drums have expanded due to heat, temporarily increasing the push rod stroke; i.e., the push rod moves to such a point that a wear indicator pops free even though there has been no excessive wearing of the shoes or linings. Other attempted solutions include devices which require that some part be inside the brake mechanism or which necessarily must be attached to a brake mounting bracket. Further attempts include devices which cannot be used when a boot or protector is used around the push rod. Many of the prior art indicators must be re-set manually.
In accordance with 37 C.F.R. .sctn.1.56 the following are disclosed: